Because of their rapid hardening, high strength, and good binding to existing concrete, magnesium silico-phosphate cements, (MSPC) and in particular ammonium magnesium silico-phosphate (monoammonium phosphate, or MAP) cements (which comprise inter alia MgO and a soluble phosphate salt) are widely used as patching mortar for roads and airport runways. While rapid hardening can be a positive characteristic in situations such as road or runway repair where minimization of downtime is a goal, too rapid hardening can be a drawback as it limits the amount of time during which the cement can be worked before it sets. In order to control the hardening time, additives have been developed, primarily to lengthen the time before the cement sets. The most frequently used retardants for these cements are based on borate salts or boric acid, which can extend the time during which the cement is workable from about 10 minutes to about half an hour (see, e.g. U.S. Pat. No. 3,960,580 and U.S. Pat. No. 7,160,383). It should be mentioned here that the amount of retardant that can be added is limited to about 1-2% w/w, which extends the workability by only 10 minutes. Larger amounts of retardant can further extend the workability, but at the expense of significant deterioration in the compressive strength (CS) of the cement after it has set.
Other retardant systems have been proposed to overcome these difficulties. For example, U.S. Pat. No. 4,786,328 discloses the use of polycarboxylic acids (e.g. citric acid) or polyphosphonic acids (e.g. nitrilotris(methylene)tris(phosphonic acid). These compounds do not significantly extend the time before the cement sets, however. U.S. Pat. No. 6,783,799 discloses the use of fluorosilicates as retardants. In this case, however, the primary means by which the set time is extended is to delay for as long as possible the mixing of the acid and base fractions of the cement mix, presumably to reduce the rate of formation of the complex hydrated salt MMgPO4·6H2O, where M is an alkali metal or NH4+. Due to the high exothermicity of the chemical reaction between the cement and added water (e.g. ΔHrxn˜−88 kcal/mol for formation of KMgPO4·6H2O), the addition of water leads to a rise in temperature, causing the process to undergo auto-acceleration. Simple fluoride salts have also been proposed as retardants for phosphate cements. For example, U.S. Pat. No. 6,458,423 teaches the use of a number of compounds including NaF and CaF2 for use as retardants for phosphate cements. There is no evidence, however, that these retardants are any more effective than the borate salts currently considered most effective. Tomic, in U.S. Pat. No. 4,758,278, discloses the use of magnesium ferrate, prepared by heating magnesium oxide particles in the presence of ferric oxide, as a retardant. While this method did succeed in approximately doubling the set time of the resulting cement, it requires an additional preparative step, and even with the use of magnesium ferrate, set times were typically no longer than those obtained by the use of borate retardants.
There thus remains a long-felt need for a straightforward method by which the rate of hardening of these cements can be controlled more precisely than by the crude methods known in the prior art.